Combine harvesters, or combines, are used to reap, thresh, and winnow or separate grain crops. In the threshing process, the grain is separated from the chaff, and in the separation process, the grain and chaff are separated into separate streams. In one common type of combine harvester, a rotor that carries devices to assist in separation, such as rasp bars and blades, sweep, etc., and rotates within a concave grating. The rasp bars help loosen and begin separating the grain from the chaff as it is pushed through the grating.
In order to increase capacity, rotary combines continue to add increasingly larger engines. However, there are limits to the ability to increase capacity in this way given the geometric shape of the machines due to road travel weight and size restrictions.